The present invention relates to structures and methods for forming electrical contact areas on semiconductor devices, whereby such devices may be connected to external circuitry. More particularly, the present invention relates to the formation of such contact areas by screening a metal ink onto the surface of a semiconductor device and subjecting the ink to a firing process in a manner similar to that utilized in thick film technology.
Numerous methods are utilized for metallizing and electrically contacting semiconductor devices. Such methods, used singly or in combination, include thermal compression bonding, vapor deposition, plasma sputtering, electrolytic plating, and electroless plating. These methods often require multiple, repetitive processing steps and the use of quite expensive materials such as gold, palladium, rhodium, platinum and silver. The plating methods raise problems of masking difficulty and poor adhesion as well as front layer metallurgical punch-through in sintering, poor conductivity, variable solderability, and poor moisture stability. The vapor deposition processes often result in a moisture sensitive contact that can degrade as a function of time in normal ambients. The sputtering processes have been difficult to make cost competitive, especially when sintering is required between depositions.
Thick film technology originated in the ceramics industry, wherein precious metal decorative inks such as platinum or gold were screened onto ceramic items and fired to achieve permanence. Thick films are defined as being within the range of 12 .mu.m (0.005 inches) to 50 .mu.m (0.002 inches). Thick film technology was introduced into electronics to provide metallization on a ceramic substrate, and later was used to fabricate multilayer chip capacitors; however, it has primarily been directed to deposition upon metal oxide (i.e., ceramic) surfaces rather than upon surfaces that are to be kept relatively free of oxides.
Recently thick film technology has been utilized to create contact areas on semiconductor devices, particularly large devices such as solar cells. Both a grid to act as a front contact and a back contact have been fabricated using conventional thick film techniques. These techniques utilize a metal ink system consisting of a metal powder, a binder and vehicle, and a metal oxide (glass) frit. Typically the metal powder has been silver, sometimes with a small amount of palladium added, and the glass frit has typically been a lead oxide, a bismuth oxide, or a borosilicate. Typical binders and vehicles have been ethyl cellulose and butyl carbitol, respectively; the binder gives the inks cohesion and the vehicle provides the necessary viscosity prior to firing. The ink is screened onto the semiconductor device in the desired pattern by conventional screening techniques, and is then fired at a temperature sufficient to cause the glass frit to become liquid. In the firing process, a portion of the metal powder is dissolved in the liquid frit, and the dissolved metal is then transported to areas of high surface energy such as the negative curvature between undissolved metal particles in contact. This effect causes grain growth and layer shrinkage, resulting in a coherent metal structure wherein the glass frit fills the interstices between cemented metal powder grains.
The above process is typically carried out in an oxidizing atmosphere to remove the binder by oxidation, and at a temperature of 400.degree. C. to 900.degree. C. depending on the melting point of the glass frit. Because it is easily oxidizable, an inexpensive metal such as copper cannot be easily used in such a process, and the surface of the semiconductor device also tends to oxidize and thus create an electrically insulating layer. If the process departs from optimum firing conditions, the glass frit is likely to segregate and migrate to the free surface, making the contact unsolderable, or to the semiconductor surface, increasing the series resistance. Efforts to improve either situation by a hydrofluoric acid dip sometimes cause the contact to fall off the semiconductor surface. Also, such contacts often lack adhesion and cohesiveness, and tend to leach off easily in conventional solders because of the high solubility of silver in tin. Additionally, it is important, particularly in the case of solar cells, that the contact withstand environmental stresses such as moisture, humidity, thermal cycling, vibration, and contamination. Some of the glass frit systems have demonstrated poor resistance to such environmental stresses, particularly moisture.
A metal frit system has been used as the liquid phase medium in a sintering process in the field of powder metal technology for the production of cemented carbides such as tungsten carbide cutting and grinding tools. In this process a metal powder such as tungsten carbide powder is mixed with about six percent by weight of cobalt powder in addition to vehicle and binder. The mixture is then pressed to shape, prefired in hydrogen to remove the binder, and liquid sintered in a vacuum at approximately 1400.degree. C. The cobalt powder, whose melting point is depressed by the dissolved carbon of the tungsten carbide, furnishes the liquid medium. Such use of a metal frit, however, has heretofore been unknown in the field of thick film technology.